Time travel is the concept of moving between different points in time in a manner analogous to moving between different points in space, either sending objects (or in some cases just information) backwards in time to some moment before the present, or sending objects forward from the present to the future without the need to experience the intervening period (at least not at the normal rate).

Although time travel has been a common plot device in fiction since the 19th century, and one-way travel into the future is arguably possible, given the phenomenon of time dilation based on velocity in the theory of special relativity (exemplified by the twin paradox), as well as gravitational time dilation in the theory of general relativity, it is currently unknown whether the laws of physics would allow backwards time travel.

Any technological device, whether fictional or hypothetical, that is used to achieve time travel is commonly known as a ''time machine''.

Some interpretations of time travel also suggest that an attempt to travel backwards in time might take one to a parallel universe whose history would begin to diverge from the traveler's original history after the moment the traveler arrived in the past.

Origins of the concept

700s BCE to 300s CE – ''Mahabharata''

200s to 400s CE – Talmud

720 CE – "Urashima Tarō"

1733 – Samuel Madden’s ''Memoirs of the Twentieth Century''

1771 – Louis-Sébastien Mercier’s ''L'An 2440, rêve s'il en fût jamais''

1781 – Johan Herman Wessel's ''Anno 7603''

1819 – Washington Irving's "Rip Van Winkle"

1824 – Faddey Bulgarin's "Pravdopodobnie Nebylitsi"

1836 – Alexander Veltman's ''Predki Kalimerosa''

1838 – ''Missing One's Coach: An Anachronism''

1843 – Charles Dickens’ ''A Christmas Carol''

1861 – Pierre Boitard’s ''Paris avant les hommes''

1881 – Edward Page Mitchell’s ''The Clock That Went Backward''

1887 – Enrique Gaspar y Rimbau's ''El anacronópete''

1888 – H. G. Wells' ''The Chronic Argonauts''

1889 – Mark Twain’s ''A Connecticut Yankee in King Arthur's Court''

1895 – H. G. Wells’ ''The Time Machine''

There is no widespread agreement as to which written work should be recognized as the earliest example of a time travel story, since a number of early works feature elements ambiguously suggestive of time travel. Ancient folk tales and myths sometimes involved something akin to travelling ''forward'' in time; for example, in Hindu mythology, the ''Mahabharata'' mentions the story of the King Revaita, who travels to heaven to meet the creator Brahma and is shocked to learn that many ages have passed when he returns to Earth. Another one of the earliest known stories to involve traveling forwards in time to a distant future was the Japanese tale of "Urashima Tarō", first described in the ''Nihongi'' (720). It was about a young fisherman named Urashima Taro who visits an undersea palace and stays there for three days. After returning home to his village, he finds himself three hundred years in the future, where he is long forgotten, his house in ruins, and his family long dead. Another very old example of this type of story can be found in the Talmud with the story of Honi HaM'agel who went to sleep for 70 years and woke up to a world where his grandchildren were grandparents and where all his friends and family were deceased. More recently, Washington Irving's famous 1819 story "Rip Van Winkle" deals with a similar concept, telling the tale of a man named Rip Van Winkle who takes a nap at a mountain and wakes up twenty years in the future, where he has been forgotten, his wife deceased, and his daughter grown up. Sleep was also used for time travel in Faddey Bulgarin's story "Pravdopodobnie Nebylitsi" where the protagonist wakes up in the 29th century.

Another more recent story involving travel to the future is Louis-Sébastien Mercier's ''L'An 2440, rêve s'il en fût jamais'' ("The Year 2440: A Dream If Ever There Were One"), a utopian novel in which the main character is transported to the year 2440. An extremely popular work (it went through twenty-five editions after its first appearance in 1771), the work describes the adventures of an unnamed man, who, after engaging in a heated discussion with a philosopher friend about the injustices of Paris, falls asleep and finds himself in a Paris of the future. Robert Darnton writes that "despite its self-proclaimed character of fantasy...L'An 2440 demanded to be read as a serious guidebook to the future."

Backwards time travel seems to be a more modern idea, but the origin of this notion is also somewhat ambiguous. One early story with hints of backwards time travel is ''Memoirs of the Twentieth Century'' (1733) by Samuel Madden, which is mainly a series of letters from English ambassadors in various countries to the British Lord High Treasurer, along with a few replies from the British Foreign Office, all purportedly written in 1997 and 1998 and describing the conditions of that era. However, the framing story is that these letters were actual documents given to the narrator by his guardian angel one night in 1728; for this reason, Paul Alkon suggests in his book ''Origins of Futuristic Fiction'' that "the first time-traveler in English literature is a guardian angel who returns with state documents from 1998 to the year 1728", although the book does not explicitly show how the angel obtained these documents. Alkon later qualifies this by writing, "It would be stretching our generosity to praise Madden for being the first to show a traveler arriving ''from'' the future", but also says that Madden "deserves recognition as the first to toy with the rich idea of time-travel in the form of an artifact sent backwards from the future to be discovered in the present."

In 1836 Alexander Veltman published ''Predki Kalimerosa: Aleksandr Filippovich Makedonskii'' (The forebears of Kalimeros: Alexander, son of Philip of Macedon), which has been called the first original Russian science fiction novel and the first novel to use time travel. In it the narrator rides to ancient Greece on a hippogriff, meets Aristotle, and goes on a voyage with Alexander the Great before returning to the 19th century.

In the science fiction anthology ''Far Boundaries'' (1951), the editor August Derleth identifies the short story ''"Missing One's Coach: An Anachronism"'', written for the ''Dublin Literary Magazine'' by an anonymous author in 1838, as a very early time travel story. In this story, the narrator is waiting under a tree to be picked up by a coach which will take him out of Newcastle, when he suddenly finds himself transported back over a thousand years, where he encounters the Venerable Bede in a monastery, and gives him somewhat ironic explanations of the developments of the coming centuries. It is never entirely clear whether these events actually occurred or were merely a dream—the narrator says that when he initially found a comfortable-looking spot in the roots of the tree, he sat down, "and as my sceptical reader will tell me, nodded and slept", but then says that he is "resolved not to admit" this explanation. A number of dreamlike elements of the story may suggest otherwise to the reader, such as the fact that none of the members of the monastery seem to be able to see him at first, and the abrupt ending where Bede has been delayed talking to the narrator and so the other monks burst in thinking that some harm has come to him, and suddenly the narrator finds himself back under the tree in the present (August 1837), with his coach having just passed his spot on the road, leaving him stranded in Newcastle for another night.

Charles Dickens' 1843 book ''A Christmas Carol'' is considered by some to be one of the first depictions of time travel, as the main character, Ebenezer Scrooge, is transported to Christmases past, present and yet to come. These might be considered mere visions rather than actual time travel, though, since Scrooge only viewed each time period passively, unable to interact with them.

A clearer example of time travel is found in the popular 1861 book ''Paris avant les hommes (Paris before Men)'' by the French botanist and geologist Pierre Boitard, published posthumously. In this story the main character is transported into the prehistoric past by the magic of a "lame demon" (a French pun on Boitard's name), where he encounters such extinct animals as a Plesiosaur, as well as Boitard's imagined version of an apelike human ancestor, and is able to actively interact with some of them.

Another clear early example of time travel in fiction is the short story '''' by Edward Page Mitchell, which appeared in the ''New York Sun'' in 1881.

Mark Twain's ''A Connecticut Yankee in King Arthur's Court'' (1889), in which the protagonist finds himself in the time of King Arthur after a fight in which he is hit with a sledge hammer, was another early time travel story which helped bring the concept to a wide audience, and was also one of the first stories to show history being changed by the time traveler's actions.

The first time travel story to feature time travel by means of a time ''machine'' was Enrique Gaspar y Rimbau's 1887 book ''El Anacronópete''. This idea gained popularity with the H. G. Wells story ''The Time Machine'', published in 1895 (preceded by a less influential story of time travel Wells wrote in 1888, titled ''The Chronic Argonauts''), which also featured a time machine and which is often seen as an inspiration for all later science fiction stories featuring time travel, using a vehicle that allows an operator to travel purposefully and selectively. The term "''time machine''", coined by Wells, is now universally used to refer to such a vehicle.

Since that time, both science and fiction (see Time travel in fiction) have expanded on the concept of time travel.

Theory

Some theories, most notably special and general relativity, suggest that suitable geometries of spacetime, or specific types of motion in space, might allow time travel into the past and future if these geometries or motions are possible. In technical papers, physicists generally avoid the commonplace language of "moving" or "traveling" through time ('movement' normally refers only to a change in spatial position as the time coordinate is varied), and instead discuss the possibility of closed timelike curves, which are worldlines that form closed loops in spacetime, allowing objects to return to their own past. There are known to be solutions to the equations of general relativity that describe spacetimes which contain closed timelike curves (such as Gödel spacetime), but the physical plausibility of these solutions is uncertain.

Relativity states that if one were to move away from the Earth at relativistic velocities and return, more time would have passed on Earth than for the traveler, so in this sense it is accepted that relativity allows "travel into the future" (according to relativity there is no single objective answer to how much time has 'really' passed between the departure and the return, but there is an objective answer to how much proper time has been experienced by both the Earth and the traveler, i.e. how much each has aged; ''See'' twin paradox). On the other hand, many in the scientific community believe that backwards time travel is highly unlikely. Any theory which would allow time travel would require that problems of causality be resolved. The classic example of a problem involving causality is the "grandfather paradox": what if one were to go back in time and kill one's own grandfather before one's father was conceived? But some scientists believe that paradoxes can be avoided, either by appealing to the Novikov self-consistency principle or to the notion of branching parallel universes (see the 'Paradoxes' section below).

Tourism in time

Stephen Hawking once suggested that the absence of tourists from the future constitutes an argument against the existence of time travel—a variant of the Fermi paradox. Of course this would not prove that time travel is physically impossible, since it might be that time travel is physically possible but that it is never in fact developed (or is cautiously never used); and even if it is developed, Hawking notes elsewhere that time travel might only be possible in a region of spacetime that is warped in the correct way, and that if we cannot create such a region until the future, then time travelers would not be able to travel back before that date, so "This picture would explain why we haven't been over run by tourists from the future." Carl Sagan also once suggested the possibility that time travelers could be here, but are disguising their existence or are not recognized as time travelers.

General relativity

However, the theory of general relativity does suggest scientific grounds for thinking backwards time travel could be possible in certain unusual scenarios, although arguments from semiclassical gravity suggest that when quantum effects are incorporated into general relativity, these loopholes may be closed. These semiclassical arguments led Hawking to formulate the chronology protection conjecture, suggesting that the fundamental laws of nature prevent time travel, but physicists cannot come to a definite judgment on the issue without a theory of quantum gravity to join quantum mechanics and general relativity into a completely unified theory.

In physics

Time travel to the past is theoretically allowed using the following methods:

Travelling faster than the speed of light

The use of cosmic strings and black holes

Wormholes and Alcubierre drive

Via faster-than-light (FTL) travel

If one were able to move information or matter from one point to another faster than light, then according to special relativity, there would be some inertial frame of reference in which the signal or object was moving backward in time. This is a consequence of the relativity of simultaneity in special relativity, which says that in some cases different reference frames will disagree on whether two events at different locations happened "at the same time" or not, and they can also disagree on the order of the two events (technically, these disagreements occur when the spacetime interval between the events is 'space-like', meaning that neither event lies in the future light cone of the other). If one of the two events represents the sending of a signal from one location and the second event represents the reception of the same signal at another location, then as long as the signal is moving at the speed of light or slower, the mathematics of simultaneity ensures that all reference frames agree that the transmission-event happened before the reception-event.

However, in the case of a hypothetical signal moving faster than light, there would always be some frames in which the signal was received before it was sent, so that the signal could be said to have moved backwards in time. And since one of the two fundamental postulates of special relativity says that the laws of physics should work the same way in every inertial frame, then if it is possible for signals to move backwards in time in any one frame, it must be possible in all frames. This means that if observer A sends a signal to observer B which moves FTL (faster than light) in A's frame but backwards in time in B's frame, and then B sends a reply which moves FTL in B's frame but backwards in time in A's frame, it could work out that A receives the reply before sending the original signal, a clear violation of causality in ''every'' frame. An illustration of such a scenario using spacetime diagrams can be found here.

According to special relativity, it would take an infinite amount of energy to accelerate a slower-than-light object to the speed of light. Although relativity does not forbid the theoretical possibility of tachyons which move faster than light at all times, when analyzed using quantum field theory, it seems that it would not actually be possible to use them to transmit information faster than light, and that there is no evidence for their existence.

Special spacetime geometries

The general theory of relativity extends the special theory to cover gravity, illustrating it in terms of curvature in spacetime caused by mass-energy and the flow of momentum. General relativity describes the universe under a system of field equations, and there exist solutions to these equations that permit what are called "closed time-like curves," and hence time travel into the past. The first of these was proposed by Kurt Gödel, a solution known as the Gödel metric, but his (and many others') example requires the universe to have physical characteristics that it does not appear to have. Whether general relativity forbids closed time-like curves for all realistic conditions is unknown.

Using wormholes

''Wormholes'' are a hypothetical warped spacetime which are also permitted by the Einstein field equations of general relativity, although it would be impossible to travel through a wormhole unless it were what is known as a traversable wormhole.

A proposed time-travel machine using a traversable wormhole would (hypothetically) work in the following way: One end of the wormhole is accelerated to some significant fraction of the speed of light, perhaps with some advanced propulsion system, and then brought back to the point of origin. Alternatively, another way is to take one entrance of the wormhole and move it to within the gravitational field of an object that has higher gravity than the other entrance, and then return it to a position near the other entrance. For both of these methods, time dilation causes the end of the wormhole that has been moved to have aged less than the stationary end, as seen by an external observer; however, time connects differently ''through'' the wormhole than ''outside'' it, so that synchronized clocks at either end of the wormhole will always remain synchronized as seen by an observer passing through the wormhole, no matter how the two ends move around. This means that an observer entering the accelerated end would exit the stationary end when the stationary end was the same age that the accelerated end had been at the moment before entry; for example, if prior to entering the wormhole the observer noted that a clock at the accelerated end read a date of 2007 while a clock at the stationary end read 2012, then the observer would exit the stationary end when its clock also read 2007, a trip backwards in time as seen by other observers outside. One significant limitation of such a time machine is that it is only possible to go as far back in time as the initial creation of the machine; in essence, it is more of a path through time than it is a device that itself moves through time, and it would not allow the technology itself to be moved backwards in time. This could provide an alternative explanation for Hawking's observation: a time machine will be built someday, but has not yet been built, so the tourists from the future cannot reach this far back in time.

According to current theories on the nature of wormholes, construction of a traversable wormhole would require the existence of a substance with negative energy (often referred to as "exotic matter") . More technically, the wormhole spacetime requires a distribution of energy that violates various energy conditions, such as the null energy condition along with the weak, strong, and dominant energy conditions. However, it is known that quantum effects can lead to small measurable violations of the null energy condition, and many physicists believe that the required negative energy may actually be possible due to the Casimir effect in quantum physics. Although early calculations suggested a very large amount of negative energy would be required, later calculations showed that the amount of negative energy can be made arbitrarily small.

In 1993, Matt Visser argued that the two mouths of a wormhole with such an induced clock difference could not be brought together without inducing quantum field and gravitational effects that would either make the wormhole collapse or the two mouths repel each other. Because of this, the two mouths could not be brought close enough for causality violation to take place. However, in a 1997 paper, Visser hypothesized that a complex "Roman ring" (named after Tom Roman) configuration of an N number of wormholes arranged in a symmetric polygon could still act as a time machine, although he concludes that this is more likely a flaw in classical quantum gravity theory rather than proof that causality violation is possible.

Other approaches based on general relativity

Another approach involves a dense spinning cylinder usually referred to as a Tipler cylinder, a GR solution discovered by Willem Jacob van Stockum in 1936 and Kornel Lanczos in 1924, but not recognized as allowing closed timelike curves until an analysis by Frank Tipler in 1974. If a cylinder is infinitely long and spins fast enough about its long axis, then a spaceship flying around the cylinder on a spiral path could travel back in time (or forward, depending on the direction of its spiral). However, the density and speed required is so great that ordinary matter is not strong enough to construct it. A similar device might be built from a cosmic string, but none are known to exist, and it does not seem to be possible to create a new cosmic string.

Physicist Robert Forward noted that a naïve application of general relativity to quantum mechanics suggests another way to build a time machine. A heavy atomic nucleus in a strong magnetic field would elongate into a cylinder, whose density and "spin" are enough to build a time machine. Gamma rays projected at it might allow information (not matter) to be sent back in time; however, he pointed out that until we have a single theory combining relativity and quantum mechanics, we will have no idea whether such speculations are nonsense.

A more fundamental objection to time travel schemes based on rotating cylinders or cosmic strings has been put forward by Stephen Hawking, who proved a theorem showing that according to general relativity it is impossible to build a time machine of a special type (a "time machine with the compactly generated Cauchy horizon") in a region where the weak energy condition is satisfied, meaning that the region contains no matter with negative energy density (exotic matter). Solutions such as Tipler's assume cylinders of infinite length, which are easier to analyze mathematically, and although Tipler suggested that a finite cylinder might produce closed timelike curves if the rotation rate were fast enough, he did not prove this. But Hawking points out that because of his theorem, "it can't be done with positive energy density everywhere! I can prove that to build a finite time machine, you need negative energy." This result comes from Hawking's 1992 paper on the chronology protection conjecture, where he examines "the case that the causality violations appear in a finite region of spacetime without curvature singularities" and proves that "[t]here will be a Cauchy horizon that is compactly generated and that in general contains one or more closed null geodesics which will be incomplete. One can define geometrical quantities that measure the Lorentz boost and area increase on going round these closed null geodesics. If the causality violation developed from a noncompact initial surface, the averaged weak energy condition must be violated on the Cauchy horizon." However, this theorem does not rule out the possibility of time travel 1) by means of time machines with the non-compactly generated Cauchy horizons (such as the Deutsch-Politzer time machine) and 2) in regions which contain exotic matter (which would be necessary for traversable wormholes or the Alcubierre drive). Because the theorem is based on general relativity, it is also conceivable a future theory of quantum gravity which replaced general relativity would allow time travel even without exotic matter (though it is also possible such a theory would place even more restrictions on time travel, or rule it out completely as postulated by Hawking's chronology protection conjecture).

Experiments carried out

Certain experiments carried out give the impression of reversed causality but are interpreted in a different way by the scientific community. For example, in the delayed choice quantum eraser experiment performed by Marlan Scully, pairs of entangled photons are divided into "signal photons" and "idler photons", with the signal photons emerging from one of two locations and their position later measured as in the double slit experiment, and depending on how the idler photon is measured, the experimenter can either learn which of the two locations the signal photon emerged from or "erase" that information. Even though the signal photons can be measured before the choice has been made about the idler photons, the choice seems to retroactively determine whether or not an interference pattern is observed when one correlates measurements of idler photons to the corresponding signal photons. However, since interference can only be observed after the idler photons are measured and they are correlated with the signal photons, there is no way for experimenters to tell what choice will be made in advance just by looking at the signal photons, and under most interpretations of quantum mechanics the results can be explained in a way that does not violate causality.

The experiment of Lijun Wang might also show causality violation since it made it possible to send packages of waves through a bulb of caesium gas in such a way that the package appeared to exit the bulb 62 nanoseconds before its entry. But a wave package is not a single well-defined object but rather a sum of multiple waves of different frequencies (''see'' Fourier analysis), and the package can appear to move faster than light or even backwards in time even if none of the pure waves in the sum do so. This effect cannot be used to send any matter, energy, or information faster than light, so this experiment is understood not to violate causality either.

The physicists Günter Nimtz and Alfons Stahlhofen, of the University of Koblenz, claim to have violated Einstein's theory of relativity by transmitting photons faster than the speed of light. They say they have conducted an experiment in which microwave photons – energetic packets of light – traveled "instantaneously" between a pair of prisms that had been moved up to apart, using a phenomenon known as quantum tunneling. Nimtz told New Scientist magazine: "For the time being, this is the only violation of special relativity that I know of." However, other physicists say that this phenomenon does not allow information to be transmitted faster than light. Aephraim Steinberg, a quantum optics expert at the University of Toronto, Canada, uses the analogy of a train traveling from Chicago to New York, but dropping off train cars at each station along the way, so that the center of the train moves forward at each stop; in this way, the center of the train exceeds the speed of any of the individual cars.

Some physicists have performed experiments which attempted to show causality violations, but so far without success. The Space-time Twisting by Light (STL) experiment run by physicist Ronald Mallett attempts to observe a violation of causality when a neutron is passed through a circle made up of a laser whose path has been twisted by passing it through a photonic crystal. Mallett has some physical arguments that suggest that closed timelike curves would become possible through the center of a laser which has been twisted into a loop. However, other physicists dispute his arguments (''see'' objections).

Shengwang Du claims in a peer reviewed journal to have observed single photons' precursors, saying that they travel no faster than c in a vacuum. His experiment involved slow light as well as passing light through a vacuum. He generated two single photons, passing one through rubidium atoms that had been cooled with a laser (thus slowing the light) and passing one through a vacuum. Both times, apparently, the precursors preceded the photons' main bodies, and the precursor traveled at c in a vacuum. According to Du, this implies that there is no possibility of light traveling faster than c (and, thus, violating causality). (Some members of the media took this as an indication of proof that time travel was impossible.)

Non-physics-based experiments

Several experiments have been carried out to try to entice future humans, who might invent time travel technology, to come back and demonstrate it to people of the present time. Events such as Perth's Destination Day (2005) or MIT's Time Traveler Convention heavily publicized permanent "advertisements" of a meeting time and place for future time travelers to meet. Back in 1982, a group in Baltimore, MD., identifying itself as the Krononauts, hosted an event of this type welcoming visitors from the future. These experiments only stood the possibility of generating a positive result demonstrating the existence of time travel, but have failed so far—no time travelers are known to have attended either event. It is hypothetically possible that future humans have traveled back in time, but have traveled back to the meeting time and place in a parallel universe. Another factor is that for all the time travel devices considered under current physics (such as those that operate using wormholes), it is impossible to travel back to before the time machine was actually made.

Time travel to the future in physics

There are various ways in which a person could "travel into the future" in a limited sense: the person could set things up so that in a small amount of his own subjective time, a large amount of subjective time has passed for other people on Earth. For example, an observer might take a trip away from the Earth and back at relativistic velocities, with the trip only lasting a few years according to the observer's own clocks, and return to find that thousands of years had passed on Earth. It should be noted, though, that according to relativity there is no objective answer to the question of how much time "really" passed during the trip; it would be equally valid to say that the trip had lasted only a few years or that the trip had lasted thousands of years, depending on your choice of reference frame.

This form of "travel into the future" is theoretically allowed (and has been demonstrated at very small time scales) using the following methods:

Using velocity-based time dilation under the theory of special relativity, for instance:

Traveling at almost the speed of light to a distant star, then slowing down, turning around, and traveling at almost the speed of light back to Earth (see the Twin paradox)

Using gravitational time dilation under the theory of general relativity, for instance:

* Residing inside of a hollow, high-mass object;

* Residing just outside of the event horizon of a black hole, or sufficiently near an object whose mass or density causes the gravitational time dilation near it to be larger than the time dilation factor on Earth.

Additionally, it might be possible to see the distant future of the Earth using methods which do not involve relativity at all, although it is even more debatable whether these should be deemed a form of "time travel":

Hibernation

Suspended animation

Time dilation

''Time dilation'' is permitted by Albert Einstein's special and general theories of relativity. These theories state that, relative to a given observer, time passes more slowly for bodies moving quickly relative to that observer, or bodies that are deeper within a gravity well. For example, a clock which is moving relative to the observer will be measured to run slow in that observer's rest frame; as a clock approaches the speed of light it will almost slow to a stop, although it can never quite reach light speed so it will never completely stop. For two clocks moving inertially (not accelerating) relative to one another, this effect is reciprocal, with each clock measuring the other to be ticking slower. However, the symmetry is broken if one clock accelerates, as in the twin paradox where one twin stays on Earth while the other travels into space, turns around (which involves acceleration), and returns—in this case both agree the traveling twin has aged less. General relativity states that time dilation effects also occur if one clock is deeper in a gravity well than the other, with the clock deeper in the well ticking more slowly; this effect must be taken into account when calibrating the clocks on the satellites of the Global Positioning System, and it could lead to significant differences in rates of aging for observers at different distances from a black hole.

It has been calculated that, under general relativity, a person could travel forward in time at a rate four times that of distant observers by residing inside a spherical shell with a diameter of 5 meters and the mass of Jupiter. For such a person, every one second of their "personal" time would correspond to four seconds for distant observers. Of course, squeezing the mass of a large planet into such a structure is not expected to be within our technological capabilities in the near future.

There is a great deal of experimental evidence supporting the validity of equations for velocity-based time dilation in special relativity and gravitational time dilation in general relativity. However, with current technologies it is only possible to cause a human traveller to age less than companions on Earth by a very small fraction of a second, the current record being about 20 milliseconds for the cosmonaut Sergei Avdeyev.

Time perception

Time perception can be apparently sped up for living organisms through hibernation, where the body temperature and metabolic rate of the creature is reduced. A more extreme version of this is suspended animation, where the rates of chemical processes in the subject would be severely reduced.

Time dilation and suspended animation only allow "travel" to the future, never the past, so they do not violate causality, and it's debatable whether they should be called time travel. However time dilation can be viewed as a better fit for our understanding of the term "time travel" than suspended animation, since with time dilation less time actually does pass for the traveler than for those who remain behind, so the traveler can be said to have reached the future faster than others, whereas with suspended animation this is not the case.

Other ideas from mainstream physics

=== Paradoxes === The Novikov self-consistency principle and calculations by Kip S. Thorne indicate that simple masses passing through time travel wormholes could never engender paradoxes—there are ''no'' initial conditions that lead to paradox once time travel is introduced. If his results can be generalized, they would suggest, curiously, that none of the supposed paradoxes formulated in time travel stories can actually be formulated at a precise physical level: that is, that ''any'' situation you can set up in a time travel story turns out to permit ''many'' consistent solutions. The circumstances might, however, turn out to be almost unbelievably strange.

Parallel universes might provide a way out of paradoxes. Everett's many-worlds interpretation (MWI) of quantum mechanics suggests that all possible quantum events can occur in mutually exclusive histories. These alternate, or parallel, histories would form a branching tree symbolizing all possible outcomes of any interaction. If all possibilities exist, any paradoxes could be explained by having the paradoxical events happening in a different universe. This concept is most often used in science-fiction, but some physicists such as David Deutsch have suggested that if time travel is possible and the MWI is correct, then a time traveler should indeed end up in a different history than the one he started from. On the other hand, Stephen Hawking has argued that even if the MWI is correct, we should expect each time traveler to experience a single self-consistent history, so that time travelers remain within their own world rather than traveling to a different one. And the physicist Allen Everett argued that Deutsch's approach "involves modifying fundamental principles of quantum mechanics; it certainly goes beyond simply adopting the MWI." Everett also argues that even if Deutsch's approach is correct, it would imply that any macroscopic object composed of multiple particles would be split apart when traveling back in time through a wormhole, with different particles emerging in different worlds.

Daniel Greenberger and Karl Svozil proposed that quantum theory gives a model for time travel without paradoxes. In quantum theory observation causes possible states to 'collapse' into one measured state; hence, the past observed from the present is deterministic (it has only one possible state), but the present observed from the past has many possible states until our actions cause it to collapse into one state. Our actions will then be seen to have been inevitable.

Using quantum entanglement

Quantum-mechanical phenomena such as quantum teleportation, the EPR paradox, or quantum entanglement might appear to create a mechanism that allows for faster-than-light (FTL) communication or time travel, and in fact some interpretations of quantum mechanics such as the Bohm interpretation presume that some information is being exchanged between particles instantaneously in order to maintain correlations between particles. This effect was referred to as "spooky action at a distance" by Einstein.

Nevertheless, the fact that causality is preserved in quantum mechanics is a rigorous result in modern quantum field theories, and therefore modern theories do not allow for time travel or FTL communication. In any specific instance where FTL has been claimed, more detailed analysis has proven that to get a signal, some form of classical communication must also be used. The no-communication theorem also gives a general proof that quantum entanglement cannot be used to transmit information faster than classical signals. The fact that these quantum phenomena apparently do ''not'' allow FTL time travel is often overlooked in popular press coverage of quantum teleportation experiments. How the rules of quantum mechanics work to preserve causality is an active area of research.

Philosophical understandings of time travel

Theories of time travel are riddled with questions about causality and paradoxes. Compared to other fundamental concepts in modern physics, time is still not understood very well. Philosophers have been theorizing about the nature of time since the era of the ancient Greek philosophers and earlier. Some philosophers and physicists who study the nature of time also study the possibility of time travel and its logical implications. The probability of paradoxes and their possible solutions are often considered.

For more information on the philosophical considerations of time travel, consult the work of David Lewis or Ted Sider. For more information on physics-related theories of time travel, consider the work of Kurt Gödel (especially his theorized universe) and Lawrence Sklar.

Presentism vs. eternalism

The relativity of simultaneity in modern physics favors the philosophical view known as eternalism or four-dimensionalism (Sider, 2001), in which physical objects are either temporally extended space-time worms, or space-time worm stages, and this view would be favored further by the possibility of time travel (Sider, 2001). Eternalism, also sometimes known as "block universe theory", builds on a standard method of modeling time as a dimension in physics, to give time a similar ontology to that of space (Sider, 2001). This would mean that time is just another dimension, that future events are "already there", and that there is no objective flow of time. This view is disputed by Tim Maudlin in his ''The Metaphysics Within Physics''.

Presentism is a school of philosophy that holds that neither the future nor the past exist, and there are no non-present objects. In this view, time travel is impossible because there is no future or past to travel to. However, some 21st century presentists have argued that although past and future objects do not exist, there can still be definite truths about past and future events, and thus it is possible that a future truth about a time traveler deciding to travel back to the present date could explain the time traveler's actual appearance in the present.

The grandfather paradox

One subject often brought up in philosophical discussion of time is the idea that, if one were to go back in time, paradoxes could ensue if the time traveler were to change things. The best examples of this are the grandfather paradox and the idea of autoinfanticide. The grandfather paradox is a hypothetical situation in which a time traveler goes back in time and attempts to kill his grandfather at a time before his grandfather met his grandmother. If he did so, then his mother or father never would have been born, and neither would the time traveler himself, in which case the time traveler never would have gone back in time to kill his grandfather.

Autoinfanticide works the same way, where a traveler goes back and attempts to kill himself as an infant. If he were to do so, he never would have grown up to go back in time to kill himself as an infant.

This discussion is important to the philosophy of time travel because philosophers question whether these paradoxes make time travel impossible. Some philosophers answer the paradoxes by arguing that it might be the case that backwards time travel could be possible but that it would be impossible to actually ''change'' the past in any way, an idea similar to the proposed Novikov self-consistency principle in physics.

Theory of compossibility

David Lewis's analysis of compossibility and the implications of changing the past is meant to account for the possibilities of time travel in a one-dimensional conception of time without creating logical paradoxes. Consider Lewis’ example of Tim. Tim hates his grandfather and would like nothing more than to kill him. The only problem for Tim is that his grandfather died years ago. Tim wants so badly to kill his grandfather himself that he constructs a time machine to travel back to 1955 when his grandfather was young and kill him then. Assuming that Tim can travel to a time when his grandfather is still alive, the question must then be raised; Can Tim kill his grandfather?

For Lewis, the answer lies within the context of the usage of the word "can". Lewis explains that the word "can" must be viewed against the context of pertinent facts relating to the situation. Suppose that Tim has a rifle, years of rifle training, a straight shot on a clear day and no outside force to restrain Tim’s trigger finger. Can Tim shoot his grandfather? Considering these facts, it would appear that Tim can in fact kill his grandfather. In other words, all of the contextual facts are compossible with Tim killing his grandfather. However, when reflecting on the compossibility of a given situation, we must gather the most inclusive set of facts that we are able to.

Consider now the fact that Tim’s grandfather died in 1993 and ''not'' in 1955. This new fact about Tim’s situation reveals that him killing his grandfather is not compossible with the current set of facts. Tim cannot kill his grandfather because his grandfather died in 1993 and not when he was young. Thus, Lewis concludes, the statements "Tim doesn’t but can, because he has what it takes," and, "Tim doesn’t, and can’t, because it is logically impossible to change the past," are not contradictions, they are both true given the relevant set of facts. The usage of the word "can" is equivocal: he "can" and "can not" under different relevant facts. So what must happen to Tim as he takes aim? Lewis believes that his gun will jam, a bird will fly in the way, or Tim simply slips on a banana peel. Either way, there will be some logical force of the universe that will prevent Tim every time from killing his grandfather.

Ideas from fiction

Rules of time travel

Time travel themes in science fiction and the media can generally be grouped into two general categories (based on effect—methods are extremely varied and numerous), each of which can be further subdivided. However, there are no formal names for these two categories, so concepts rather than formal names will be used with notes regarding what categories they are placed under (''Note: These classifications do not address the method of time travel itself, i.e. how to travel through time, but instead call to attention differing rules of what happens to history.''). As used in this section, ''timeline'' refers to all physical events in history, so that in time travel stories where events can be changed, the time traveler creates a new or altered timeline. This usage of "timeline" is fairly common in time travel fiction, and is distinct from the usage of "timeline" to refer to a type of chart created by humans to illustrate a particular series of events (''see'' Timeline). This concept is also distinct from the concept of a world line, a term from Einstein's theory of relativity which refers to the entire history of a ''single'' object (usually idealized as a point particle) that forms a distinct path through through 4-dimensional spacetime.

:1. There is a single fixed history, which is self-consistent and unchangeable. In this version, everything happens on a single timeline which does not contradict itself and cannot interact with anything potentially existing outside of it.

::1.1 This can be simply achieved by applying the Novikov self-consistency principle, named after Dr. Igor Dmitrievich Novikov, Professor of Astrophysics at Copenhagen University. The principle states that the timeline is totally fixed, and any actions taken by a time traveler were part of history all along, so it is impossible for the time traveler to "change" history in any way. The time traveler's actions may be the ''cause'' of events in their own past though, which leads to the potential for circular causation and the predestination paradox; for examples of circular causation, see Robert A. Heinlein's story "By His Bootstraps". The Novikov self-consistency principle proposes that the local laws of physics in a region of spacetime containing time travelers cannot be any different from the local laws of physics in any other region of spacetime.

::1.2 Alternatively, new physical laws take effect regarding time travel that thwarts attempts to change the past (contradicting the assumption mentioned in 1.1 above that the laws that apply to time travelers are the same ones that apply to everyone else). These new physical laws can be as unsubtle as to reject time travelers who travel to the past to change it by pulling them back to the point from when they came as Michael Moorcock's ''The Dancers at the End of Time'' or where the traveler is rendered a noncorporeal phantom unable to physically interact with the past such as in some Pre-Crisis Superman stories and Michael Garrett's "Brief Encounter" in ''Twilight Zone Magazine'' May 1981.

:2. History is flexible and is subject to change (''Plastic Time'')

::2.1 ''Changes to history are easy and can impact the traveler, the world, or both'' :::Examples include ''Doctor Who'' and the ''Back to the Future'' trilogy. In some cases, any resulting paradoxes can be devastating, threatening the very existence of the universe. In other cases the traveler simply cannot return home. The extreme version of this (''Chaotic Time'') is that history is ''very'' sensitive to changes with even small changes having large impacts such as in Ray Bradbury's "A Sound of Thunder".

::2.2 ''History is change resistant in direct relationship to the importance of the event'' ie. small trivial events can be readily changed but large ones take great effort. :::In the ''Twilight Zone'' episode "Back There" a traveler tries to prevent the assassination of President Lincoln and fails, but his actions have made subtle changes to the ''status quo'' in his own time (e.g. a man who had been the butler of his gentleman's club is now a rich tycoon). :::In the 2002 remake of ''The Time Machine'', it is explained via a vision why Hartdegen could not save his sweetheart Emma—doing so would have resulted in his never developing the time machine he used to try and save her. :::In ''The Saga of Darren Shan'', major events in the past cannot be changed, but their details can alter while providing the same outcome. Under this model, if a time traveler were to go back in time and kill Hitler, another Nazi would simply take his place and commit his same actions, leaving the broader course of history unchanged. :::In the ''Doctor Who'' episode ''The Waters of Mars'', Captain Adelaide Brooke's death on Mars is the most singular catalyst of human travel outside the solar system. At first, the Doctor realizes her death is a "fixed point in time" and does not intervene, but later defies this rule and transports her and her crew to Earth. Rather than allow human history to change, Captain Brooke commits suicide on Earth, leaving history mostly unchanged.

:3. Alternate timelines. In this version of time travel, there are multiple coexisting alternate histories, so that when the traveler goes back in time, he/she ends up in a new timeline where historical events can differ from the timeline he/she came from, but her original timeline does not cease to exist (this means the grandfather paradox can be avoided since even if the time traveler's grandfather is killed at a young age in the new timeline, he still survived to have children in the original timeline, so there is still a causal explanation for the traveler's existence). Time travel may actually ''create'' a new timeline that diverges from the original timeline at the moment the time traveler appears in the past, or the traveler may arrive in an already existing parallel universe (though unless the parallel universe's history was identical to the time traveler's history up until the point where the time traveler appeared, it is questionable whether the latter version qualifies as 'time travel').

::James P. Hogan's ''The Proteus Operation'' fully explains parallel universe time travel in chapter 20 where it has Einstein explaining that all the outcomes already exist and all time travel does is change which already existing branch you will experience.

::Though ''Star Trek'' has a long tradition of using the 2.1 mechanic, as seen in "The City on the Edge of Forever", "Tomorrow is Yesterday", "Time and Again", "Future's End", "Before and After", "Endgame" and as late as Enterprise's Temporal Cold War, "Parallels" had an example of what Data called "quantum realities." His exact words on the matter were "But there is a theory in quantum physics that all possibilities that can happen do happen in alternate quantum realities," suggesting the writers were thinking of the many-worlds interpretation of quantum mechanics.

::Michael Crichton's novel ''Timeline'' takes the approach that all time travel really is travel to an already existing parallel universe where time passes at a slower rate than our own but actions in any of these parallel universes may have already occurred in our past. It is unclear from the novel if any sizable change in events of these parallel universe can be made.

::In the Homeline setting of ''GURPS Infinite Worlds'' there are echos—parallel universes at an early part of Homeline's history but changes to their history do not affect Homeline's history. However tampering with their history can cause them to shift quanta making access harder if not impossible.

::A type of story which could be placed in this category is one where the alternative version of the past lies not in some other dimension, but simply at a distant location in space or a future period of time that replicates conditions in the traveler's past. For example, in a Futurama episode called ''The Late Philip J. Fry'', the professor designed a forward-only time travel device. Trapped in the future, he and two colleagues travel forward all the way to the end of the universe, at which point they witness a new Big Bang which gives rise to a new universe whose history mirrors their own history. Then they continue to go forward until they reach the exact time of their initial departure. Although this journey is not exactly a backward time travel, the final result is the same.

::In the Japanese manga, ''Dragon Ball Z'', the character Trunks travels back in time to warn the characters of their deaths soon to come. This does not change his time line, only creates a new one in which they do not die. Soon two of the characters destroy the lab where the monster Cell is being created, stopping him from absorbing the androids, creating a third time line. Later it is revealed that Trunks is killed by Cell in the future, then travels to three years before any of the events occurs, which creates a fourth time line. No matter what any character does in the past, their own original time line is unchanged.

Immutable timelines

Time travel in a type 1 universe does not allow paradoxes such as the grandfather paradox to occur, where one deduces both a conclusion and its opposite (in the case of the grandfather paradox, one can start with the premise of the time traveler killing his grandfather, and reach the conclusion that the time traveler will ''not'' be able to kill his grandfather since he was never born) though it can allow other paradoxes to occur.

In 1.1, the Novikov self-consistency principle asserts that the existence of a method of time travel constrains events to remain self-consistent. This will cause any attempt to violate such consistency to fail, even if seemingly extremely improbable events are required.

:Example: You have a device that can send a single bit of information back to itself at a precise moment in time. You receive a bit at 10:00:00 p.m., then no bits for thirty seconds after that. If you send a bit back to 10:00:00 p.m., everything works fine. However, if you try to send a bit to 10:00:15 p.m. (a time at which no bit was received), your transmitter will mysteriously fail. Or your dog will distract you for fifteen seconds. Or your transmitter will appear to work, but as it turns out your receiver failed at exactly 10:00:15 p.m., etc. Examples of this kind of universe are found in Robert Forward's novel ''Timemaster'', the ''Twilight Zone'' episode "No Time Like the Past", and the 1980 Jeannot Szwarc film ''Somewhere In Time'' (based on Richard Matheson's novel ''Bid Time Return'').

In 1.2, time travel is constrained to prevent paradox. How this occurs is dependent on whether interaction with the past is possible.

If interaction with the past is possible and one attempts to make a paradox, one undergoes involuntary or uncontrolled time travel. In the time-travel stories of Connie Willis, time travelers encounter "slippage" which prevents them from either reaching the intended time or translates them a sufficient distance from their destination at the intended time, as to prevent any paradox from occurring.

:Example: A man who travels into the past with intentions to kill Hitler finds himself on a Montana farm in late April 1945.

In the The Dancers at the End of Time series, Michael Moorcock invented a plot device called the Morphail Effect. This causes a time traveler to be ejected from the time in which he or she is about to cause a paradox.

:Example 1: a man from the End of Time period travels to the past and is executed. Instead of dying (which would cause a paradox), he experiences a return to the End of Time :Example 2: time travelers sometimes visit the End of Time from their own epochs in the past. Those that attempt to return to their own period are likely to reappear inadvertently at the End of Time.

The general consequences are that time travel to the traveler's past is difficult, and many time travelers find themselves adventuring deeper and deeper into their future.

If interaction with the past is ''not'' possible then the traveler simply becomes an invisible insubstantial phantom unable to interact with the past as in the case of James Harrigan in Michael Garrett's "Brief Encounter".

While a Type 1 universe will prevent a grandfather paradox it doesn't prevent paradoxes in other aspects of physics such as the predestination paradox and the ontological paradox (''GURPS Infinite Worlds'' calls this "Free Lunch Paradox").

The predestination paradox is where the traveler's actions create some type of causal loop, in which some event A in the future helps cause event B in the past via time travel, and the event B in turn is one of the causes of A. For instance, a time traveler might go back to investigate a specific historical event like the Great Fire of London, and their actions in the past could then inadvertently end up being the original cause of that very event.

Examples of this kind of causal loop are found in Robert Forward's novel ''Timemaster'', the ''Twilight Zone'' episode "No Time Like the Past", EC Comics stories like "Man who was Killed in Time" (Weird Science #5), "Why Papa Left Home" (Weird Science #11), "Only Time will Tell" (Weird Fantasy #1), "The Connection" (Weird Fantasy #9), "Skeleton Key" (Weird Fantasy #16), and "Counter Clockwise" (Weird Fantasy #18), the 1980 Jeannot Szwarc film ''Somewhere In Time'' (based on Richard Matheson's novel ''Bid Time Return''), the Michael Moorcock novel ''Behold the Man'', and ''Harry Potter and the Prisoner of Azkaban''. It is also featured in 1972's ''Doctor Who'', in the three part ''The Day of the Daleks'', where three freedom fighters from the future attempt to kill a British diplomat they believe responsible for World War Three, and the subsequent easy conquest of Earth by the Daleks. In the future they were taught an explosion at the diplomat's (Sir Reginald Styles) mansion with foreign delegates inside caused the nations of the world to attack each other. The Doctor (Jon Pertwee), figures out that ''they'' caused the explosion all along by way of a temporal paradox.

In the 2006 crime thriller ''Déjà Vu'' there appears to be causal loops, as Agent Doug Carlin decides to send a message back in time to save his partner's life, but this will eventually cause his death. Later in the movie, tough, Carlin is able to change events and create an alternate reality. This apparent paradox can be explained by multiple previous unseen time travels in a type 3 universe.

In the videogame ''Escape from Monkey Island'' there's a section in which the player, controlling Guybrush Threepwood, gets some items from his future self in the Swamp of Time. Soon after that, he will become the future Guybrush and will have to give the items to his past self in the same order. This is an example of causal loop because those items were created purely from the time travel. Anyway if the player doesn't repeat every action properly, will cause a paradox that sends Guybrush back to the entrance of the swamp, implying a type 1.2 universe.

The Novikov self-consistency principle can also result in an ontological paradox (also known as the knowledge or information paradox) where the very existence of some object or information is a time loop. ''GURPS Infinite Worlds'' gives the example (from ''The Eyre Affair'') of a time traveler going to Shakespeare's time with a book of all his works. Shakespeare pressed for time simply copies the information in the book from the future. The paradox is that nobody actually writes the plays.

The philosopher Kelley L. Ross argues in "Time Travel Paradoxes" that in an ontological paradox scenario involving a physical object, there can be a violation of the second law of thermodynamics. Ross uses ''Somewhere in Time'' as an example where Jane Seymour's character gives Christopher Reeve's character a watch she has owned for many years, and when he travels back in time he gives the same watch to Jane Seymour's character 60 years in the past. As Ross states

"The watch is an impossible object. It violates the Second Law of Thermodynamics, the Law of Entropy. If time travel makes that watch possible, then time travel itself is impossible. The watch, indeed, must be ''absolutely identical'' to itself in the 19th and 20th centuries, since Reeve carries it with him from the future instantaneously into the past and bestows it on Seymour. The watch, however, cannot be identical to itself, since all the years in which it is in the possession of Seymour and then Reeve it will wear in the normal manner. It's entropy will increase. The watch carried back by Reeve will be more worn that the watch that would have been acquired by Seymour."

On the other hand, the second law of thermodynamics is understood by modern physicists to be a statistical law rather than an absolute one, so spontaneous reversals of entropy or failure to increase in entropy are not impossible, just improbable (see for example the fluctuation theorem). In addition, the second law of thermodynamics only states that entropy should increase in systems which are isolated from interactions with the external world, so Igor Novikov (creator of the Novikov self-consistency principle) has argued that in the case of macroscopic objects like the watch whose worldlines form closed loops, the outside world can expend energy to repair wear/entropy that the object acquires over the course of its history, so that it will be back in its original condition when it closes the loop.

==== Mutable timelines ==== Time travel in a Type 2 universe is much more complex. The biggest problem is how to explain changes in the past. One method of explanation is that once the past changes, so do the memories of all observers. This would mean that no observer would ever observe the changing of the past (because they will not remember changing the past). This would make it hard to tell whether you are in a Type 1 universe or a Type 2 universe. You could, however, infer such information by knowing if a) communication with the past were possible or b) it appeared that the time line had ''never'' been changed as a result of an action someone remembers taking, although evidence exists that other people are changing their time lines fairly often.

An example of this kind of universe is presented in ''Thrice Upon a Time'', a novel by James P. Hogan. The ''Back to the Future'' trilogy films also seem to feature a single mutable timeline (see the "Back to the Future FAQ" for details on how the writers imagined time travel worked in the movies' world). By contrast, the short story "Brooklyn Project" by William Tenn provides a sketch of life in a Type 2 world where no one even notices as the timeline changes repeatedly.

In type 2.1, attempts are being made at changing the timeline, however, all that is accomplished in the first tries is that the method in which decisive events occur is changed; final conclusions in the bigger scheme cannot be brought to a different outcome.

As an example, the movie ''Déjà Vu'' depicts a paper note sent to the past with vital information to prevent a terrorist attack. However, the vital information results in the killing of an ATF agent, but does not prevent the terrorist attack; the very same agent died in the previous version of the timeline as well, albeit under different circumstances. Finally, the timeline is changed by sending a human into the past, arguably a "stronger" measure than simply sending back a paper note, which results in preventing both a murder and the terrorist attack. As in the ''Back to the Future'' movie trilogy, there seems to be a ripple effect too as changes from the past "propagate" into the present, and people in the present have altered memory of events that occurred after the changes made to the timeline.

The science fiction writer Larry Niven suggests in his essay "The Theory and Practice of Time Travel" that in a type 2.1 universe, the most efficient way for the universe to "correct" a change is for time travel to never be discovered, and that in a type 2.2 universe, the very large (or infinite) number of time travelers from the endless future will cause the timeline to change wildly until it reaches a history in which time travel is never discovered. However, many other "stable" situations might also exist in which time travel occurs but no paradoxes are created; if the changeable-timeline universe finds itself in such a state no further changes will occur, and to the inhabitants of the universe it will appear identical to the type 1.1 scenario. This is sometimes referred to as the "Time Dilution Effect".

Few if any physicists or philosophers have taken seriously the possibility of "changing" the past except in the case of multiple universes, and in fact many have argued that this idea is logically incoherent, so the mutable timeline idea is rarely considered outside of science fiction.

Also, deciding whether a given universe is of Type 2.1 or 2.2 can not be done objectively, as the categorization of timeline-invasive measures as "strong" or "weak" is arbitrary, and up to interpretation: An observer can disagree about a measure being "weak", and might, in the lack of context, argue instead that simply a mishap occurred which then led to no effective change.

An example would be the paper note sent back to the past in the film ''Déjà Vu'', as described above. Was it a "too weak" change, or was it just a local-time alteration which had no extended effect on the larger timeline? As the universe in ''Déjà Vu'' seems not entirely immune to paradoxes (some arguably minute paradoxes do occur), both versions seem to be equally possible.

Alternate histories

In Type 3, any event that appears to have caused a paradox has instead created a new time line. The old time line remains unchanged, with the time traveler or information sent simply having vanished, never to return. A difficulty with this explanation, however, is that conservation of mass-energy would be violated for the origin timeline and the destination timeline. A possible solution to this is to have the mechanics of time travel require that mass-energy be exchanged in precise balance between past and future at the moment of travel, or to simply expand the scope of the conservation law to encompass all timelines. Some examples of this kind of time travel can be found in David Gerrold's book ''The Man Who Folded Himself'' and ''The Time Ships'' by Stephen Baxter, plus several episodes of the TV show ''Star Trek: The Next Generation'' and the android saga in the Japanese TV series Dragon Ball Z.

Gradual and instantaneous

In literature, there are two methods of time travel: #The most commonly used method of time travel in science fiction is the instantaneous movement from one point in time to another, like using the controls on a CD player to skip to a previous or next song, though in most cases, there is a machine of some sort, and some energy expended in order to make this happen (like the time-traveling De Lorean in ''Back to the Future'' or the TARDIS (Time and Relative Dimension in Space) that traveled through time in ''Doctor Who''). In some cases, there is not even the beginning of a scientific explanation for this kind of time travel; it's popular probably because it is more spectacular and makes time travel easier. The "Universal Remote" used by Adam Sandler in the movie ''Click'' works in the same manner, although only in one direction, the future. While his character Michael Newman can travel back to a previous point it is merely a playback with which he cannot interact. #In ''The Time Machine'', H.G. Wells explains that we are moving through time with a constant speed. Time travel then is, in Wells' words, "stopping or accelerating one's drift along the time-dimension, or even turning about and traveling the other way." George Pal, director of the 1960 adaptation based on Wells's classic, accordingly chose to depict time travel by employing time-lapse photography. To expand on the audio playback analogy used above, this would be like rewinding or fast forwarding an analogue audio cassette and playing the tape at a chosen point. Perhaps the oldest example of this method of time travel is in Lewis Carroll's ''Through the Looking-Glass'' (1871): the White Queen is living backwards, hence her memory is working both ways. Her kind of time travel is uncontrolled: she moves through time with a constant speed of −1 and she cannot change it. T.H. White, in the first part of his Arthurian novel ''The Once and Future King'', ''The Sword in the Stone'' (1938) used the same idea: the wizard Merlyn lives backward in time, because he was born "at the wrong end of time" and has to live backwards from the front. "Some people call it having second sight", he says. This method of gradual time travel is not as popular in modern science fiction, though a form of it does occur in the film ''Primer''.

Time travel or space-time travel

An objection that is sometimes raised against the concept of time machines in science fiction is that they ignore the motion of the Earth between the date the time machine departs and the date it returns. The idea that a traveler can go into a machine that sends him or her to 1865 and step out into the exact same spot on Earth might be said to ignore the issue that Earth is moving through space around the Sun, which is moving in the galaxy, and so on, so that advocates of this argument imagine that "realistically" the time machine should actually reappear in space far away from the Earth's position at that date. However, the theory of relativity rejects the idea of absolute time and space; in relativity there can be no universal truth about the spatial distance between events which occur at different times (such as an event on Earth today and an event on Earth in 1865), and thus no objective truth about which point in space at one time is at the "same position" that the Earth was at another time. In the theory of special relativity, which deals with situations where gravity is negligible, the laws of physics work the same way in every inertial frame of reference and therefore no frame's perspective is physically better than any other frame's, and different frames disagree about whether two events at different times happened at the "same position" or "different positions". In the theory of general relativity, which incorporates the effects of gravity, ''all'' coordinate systems are on equal footing because of a feature known as "diffeomorphism invariance".

Nevertheless, the idea that the Earth moves away from the time traveler when he takes a trip through time has been used in a few science fiction stories, such as the 2000 AD comic ''Strontium Dog'', in which Johnny Alpha uses "Time Bombs" to propel an enemy several seconds into the future, during which time the movement of the Earth causes the unfortunate victim to re-appear in space. Much earlier, Clark Ashton Smith used this form of time travel in several stories such as "''The Letter from Mohaun Los''" (1932) where the protagonist ends up on a planet millions of years in the future which "happened to occupy the same space through which Earth had passed". Other science fiction stories try to anticipate this objection and offer a rationale for the fact that the traveler remains on Earth, such as the 1957 Robert Heinlein novel ''The Door into Summer'' where Heinlein essentially handwaved the issue with a single sentence: "You stay on the world line you were on." In his 1980 novel ''The Number of the Beast'' a "continua device" allows the protagonists to dial in the coordinates of space and time and it instantly moves them there—without explaining how such a device might work.

The television series ''Seven Days'' also dealt with this problem; when the chrononaut would be 'rewinding', he would also be propelling himself backwards around the Earth's orbit, with the intention of landing at some chosen spatial location, though seldom hitting the mark precisely. In Piers Anthony's ''Bearing an Hourglass'', the potent Hourglass of the Incarnation of Time naturally moves the Incarnation in space according to the numerous movements of the globe through the solar system, the solar system through the galaxy, etc.; but by carefully negating some of the movements he can also travel in space within the limits of the planet. The television series ''Doctor Who'' avoided this issue by establishing early on in the series that the Doctor's TARDIS is able to move about in space in addition to traveling in time.

See also

Speculations

Grandfather paradox

Krasnikov tube

Ronald Mallett

Ontological paradox

Predestination paradox

Ring singularity

Retrocausality

Temporal paradox

Tipler Cylinder

Wheeler-Feynman time-symmetric theory

Claims of time travel

Chronovisor

Darren Daulton

Billy Meier

Moberly-Jourdain incident

Montauk Project

Philadelphia Experiment

Time slip

John Titor

Time travel urban legends

Fiction, humor

Chronodynamics

Thiotimoline

Time loop

Time travel in fiction

Notes

Bibliography

Nahin, Paul J. (1997). ''Time Travel: A writer's guide to the real science of plausible time travel''. Writer's Digest Books. Cincinnati, Ohio. ISBN 0-89879-748-9

External links

Black holes, Wormholes and Time Travel, a Royal Society Lecture

SF Chronophysics, a discussion of Time Travel as it relates to science fiction

On the Net: Time Travel by James Patrick Kelly

Time Travel in Flatland?

NOVA Online: Time Travel

Professor Predicts Human Time Travel This Century

Time Traveler Convention at MIT

Time Machines in Physics – almost 200 citations from 1937 through 2001

Time Travel and Modern Physics at the Stanford Encyclopedia of Philosophy

Time Travel at the Internet Encyclopedia of Philosophy

Aparta Krystian: Conventional Models of Time and Their Extensions in Science Fiction

Time travellers from the future 'could be here in weeks'

Time machine on arxiv.org

Category:Philosophy of physics Category:Time

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name	Never Shout Never
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background	group_or_band
alias	NeverShoutNever! NeverShoutNever
origin	Joplin, Missouri, United States
genre	Pop rock
years active	2007–present
label	Warner Bros.SireLoveway
associated acts	Mister OwlEatmewhileimhot!GONZO
website	nevershoutnever.com
current members	Christofer Drew Caleb Denison Taylor MacFee Hayden Kaiser
past members	Patrick Carrie Jamie Sheridan Hayden Benton Dustin Dobernig Nathan Ellison }}

Never Shout Never is an American pop rock band, formed in Joplin, Missouri in 2007. The band has released two albums and six extended plays. Their third album, Time Travel, is set to be released on September 20, 2011.

Career

Christofer Drew Ingle began making music as NeverShoutNever in September 2005. His first exposure came through the internet, where he achieved success on MySpace before issuing The Yippee EP on July 29, 2008. On July 30, 2008, he was featured on TRL, where he performed his single "Bigcitydreams." He toured with Hellogoodbye and Ace Enders in the fall of 2008.

As of November 17, 2008, the official spelling of Ingle's alias became NeverShoutNever! It had previously been listed as Never Shout Never on iTunes and his management company's site. An official statement regarding the reason behind the spelling change has not been made available. In an interview, Ingle revealed that there is no longer an exclamation point after his name and that he will spell NeverShoutNever as one word when he is feeling happy, but three words when he isn't.

Never Shout Never started touring with The Scene Aesthetic, The Honorary Title, and The Bigger Lights in late February 2009 and then toured with bands such as Forever the Sickest Kids, The Cab, We The Kings,and Mercy Mercedes, among others in spring 2009 as part of The Bamboozle Roadshow 2009. He played at both The Bamboozle Left 2009 and The Bamboozle 2009.

The Summer EP was released on June 23, 2009. The first single off the EP, titled "Happy," was released on iTunes on March 3, 2009.

It was announced on May 29, 2009, that Ingle had signed to Warner Bros. Records, ending a major-label bidding war. As part of the deal, Ingle will run and make releases on his own imprint label, Loveway Records.

The debut album from Never Shout Never, ''What Is Love?'', was produced by Butch Walker and released in January of 2010. Never Shout Never's second full length album, Harmony, was released in August of the same year.

In November 2010, Never Shout Never co-headlined the Harmony Tour, where they had fans gather can foods to help those in need. As a gift to the fans, Never Shout Never and The Maine released a live split EP. As of December 21, 2010, the split EP is available for free download on Never Shout Never's website.

The band's hometown, Joplin, Missouri, was destroyed by a tornado on May 22, 2011. Following the destruction, Christofer began a relief fund with United Way to raise $1 million for his hometown. To raise awareness about the devastation of Joplin, Christofer took video footage of the destruction and posted it on YouTube on June 1, 2011. The video, which features the song "Time Travel," encouraged viewers to donate to the relief of the town via his United Way fund

Never Shout Never's third full-length album will be entitled Time Travel and is set to be released on September 20, 2011.

Discography

Studio albums

''What Is Love?'' (Sire, 2010)

''Harmony'' (Sire, 2010)

''Time Travel'' (Sire, 2011)

Extended plays

''Demo-shmemo'' (Self released, 2008)

''The Yippee'' (Loveway, 2008)

''Me & My Uke'' (Loveway, 2009)

''The Summer'' (Loveway, 2009)

''Never Shout Never'' (Sire, 2009)

''Melody'' (Sire, 2010)

Live albums

''Never Shout Never & The Maine'' (Warner Bros., 2010)

''Love (Live)'' (Self released, 2011)

''To Pick A Town Up'' (Self released, 2011)

Compilation albums

''Year One'' (Self released, 2011)

Featured albums

''Almost Alice'' (Buena Vista, 2010)

''Punk Goes Classic Rock'' (Fearless, 2010)

Members

Current members

Christofer Drew - vocals, piano, guitar (2007-present)

Caleb Denison - drums, guitar, vocals (2009-present)

Taylor MacFee - bass, vocals (2010-present)

Hayden Kaiser - guitar, vocals (2010-present)

Former members

Patrick Carrie - guitar, vocals (2009)

Jamie Sheridan - guitar (2009)

Hayden Benton - drums, vocals (2009)

Dustin Dobernig - piano, keyboard (2009-2010)

Nathan Ellison - percussion, drums (2009-2010)

References

External links

Never Shout Never at MTV

Never Shout Never at Purevolume

Interview with Never Shout Never on Shockhound

Interview with Never Shout Never on ''Thought Catalog''

Category:Sire Records artists Category:Musicians from Missouri Category:People from Joplin, Missouri

de:Never Shout Never es:Never Shout Never fr:Never Shout Never it:Never Shout Never nl:Never Shout Never no:NeverShoutNever! pl:NeverShoutNever pt:NeverShoutNever! ru:Never Shout Never fi:Never Shout Never sv:Never Shout Never tr:Never Shout Never

Name	Stephen Hawking
Birth name	Stephen William Hawking
Birth date	January 08, 1942
Birth place	Oxford, England, United Kingdom
Residence	United Kingdom
Nationality	British
Fields	Applied mathematicsTheoretical physicsCosmology
Workplaces	Cambridge UniversityCalifornia Institute of TechnologyPerimeter Institute for Theoretical Physics
Alma mater	Oxford UniversityCambridge University
Doctoral advisor	Dennis Sciama
Academic advisors	Robert Berman
Doctoral students	Bruce AllenRaphael BoussoFay DowkerMalcolm PerryBernard CarrGary GibbonsHarvey ReallDon PageTim PrestidgeRaymond LaflammeJulian Luttrell
Known for	Black holesTheoretical cosmologyQuantum gravityHawking radiation
Influences	Dikran TahtaAlbert Einstein
Awards
Spouse	Jane Hawking(m. 1965–1991, divorced)Elaine Mason(m. 1995–2006, divorced)
Signature	Hawkingsig.svg }}

Stephen William Hawking, CH, CBE, FRS, FRSA (born 8 January 1942) is a British theoretical physicist and cosmologist, whose scientific books and public appearances have made him an academic celebrity. He is an Honorary Fellow of the Royal Society of Arts, a lifetime member of the Pontifical Academy of Sciences, and in 2009 was awarded the Presidential Medal of Freedom, the highest civilian award in the United States.

Hawking was the Lucasian Professor of Mathematics at the University of Cambridge for 30 years, taking up the post in 1979 and retiring on 1 October 2009. He is now Director of Research at the Centre for Theoretical Cosmology in the Department of Applied Mathematics and Theoretical Physics at the University of Cambridge. He is also a Fellow of Gonville and Caius College, Cambridge and a Distinguished Research Chair at the Perimeter Institute for Theoretical Physics in Waterloo, Ontario. He is known for his contributions to the fields of cosmology and quantum gravity, especially in the context of black holes. He has also achieved success with works of popular science in which he discusses his own theories and cosmology in general; these include the runaway best seller ''A Brief History of Time'', which stayed on the British ''Sunday Times'' best-sellers list for a record-breaking 237 weeks.

Hawking's key scientific works to date have included providing, with Roger Penrose, theorems regarding gravitational singularities in the framework of general relativity, and the theoretical prediction that black holes should emit radiation, which is today known as Hawking radiation (or sometimes as Bekenstein–Hawking radiation).

Hawking has a motor neurone disease that is related to amyotrophic lateral sclerosis, a condition that has progressed over the years and has left him almost completely paralysed.

Early life

Stephen Hawking was born on 8 January 1942 to Dr. Frank Hawking, a research biologist, and Isobel Hawking. He had two younger sisters, Philippa and Mary, and an adopted brother, Edward. Though Hawking's parents were living in North London, they moved to Oxford while his mother was pregnant with Stephen, desiring a safer location for the birth of their first child. (London was under attack at the time by the Luftwaffe.) According to Hawking, a German V-2 missile struck only a few streets away.

After Hawking was born, the family moved back to London, where his father headed the division of parasitology at the National Institute for Medical Research. In 1950, Hawking and his family moved to St Albans, Hertfordshire, where he attended St Albans High School for Girls from 1950 to 1953. (At that time, boys could attend the Girls' school until the age of ten.) From the age of eleven, he attended St Albans School, where he was a good, but not exceptional, student. When asked later to name a teacher who had inspired him, Hawking named his mathematics teacher Dikran Tahta. He maintains his connection with the school, giving his name to one of the four houses and to an extracurricular science lecture series. He has visited it to deliver one of the lectures and has also granted a lengthy interview to pupils working on the school magazine, ''The Albanian''.

Hawking was always interested in science. Inspired by his mathematics teacher, he originally wanted to study the subject at university. However, Hawking's father wanted him to apply to University College, Oxford, where his father had attended. As University College did not have a mathematics fellow at that time, it would not accept applications from students who wished to read that discipline. Hawking therefore applied to read natural sciences, in which he gained a scholarship. Once at University College, Hawking specialised in physics. His interests during this time were in thermodynamics, relativity, and quantum mechanics. His physics tutor, Robert Berman, later said in ''The New York Times Magazine'':

Hawking was passing, but his unimpressive study habits resulted in a final examination score on the borderline between first and second class honours, making an "oral examination" necessary. Berman said of the oral examination:

After receiving his B.A. degree at Oxford in 1962, he stayed to study astronomy. He decided to leave when he found that studying sunspots, which was all the observatory was equipped for, did not appeal to him and that he was more interested in theory than in observation. He left Oxford for Trinity Hall, Cambridge, where he engaged in the study of theoretical astronomy and cosmology.

Career

Almost as soon as he arrived at Cambridge, he started developing symptoms of amyotrophic lateral sclerosis (ALS, known colloquially in the United States as Lou Gehrig's disease), a type of motor neurone disease which would cost him almost all neuromuscular control. During his first two years at Cambridge, he did not distinguish himself, but, after the disease had stabilised and with the help of his doctoral tutor, Dennis William Sciama, he returned to working on his PhD.

Hawking was elected as one of the youngest Fellows of the Royal Society in 1974, was created a Commander of the Order of the British Empire in 1982, and became a Companion of Honour in 1989. Hawking is a member of the Board of Sponsors of the ''Bulletin of the Atomic Scientists''.

In 1974, he accepted the Sherman Fairchild Distinguished Scholar visiting professorship at the California Institute of Technology (Caltech) to work with his friend, Kip Thorne, who was a faculty member there. He continues to have ties with Caltech, spending a month each year there since 1992.

Hawking's achievements were made despite the increasing paralysis caused by the ALS. By 1974, he was unable to feed himself or get out of bed. His speech became slurred so that he could be understood only by people who knew him well. In 1985, he caught pneumonia and had to have a tracheotomy, which made him unable to speak at all. A Cambridge scientist built a device that enables Hawking to write onto a computer with small movements of his body, and then have a voice synthesiser speak what he has typed.

Research fields

Hawking's principal fields of research are theoretical cosmology and quantum gravity.

In the late 1960s, he and his Cambridge friend and colleague, Roger Penrose, applied a new, complex mathematical model they had created from Albert Einstein's theory of general relativity. This led, in 1970, to Hawking proving the first of many singularity theorems; such theorems provide a set of sufficient conditions for the existence of a gravitational singularity in space-time. This work showed that, far from being mathematical curiosities which appear only in special cases, singularities are a fairly generic feature of general relativity.

He supplied a mathematical proof, along with Brandon Carter, Werner Israel and D. Robinson, of John Wheeler's no-hair theorem – namely, that any black hole is fully described by the three properties of mass, angular momentum, and electric charge.

Hawking also suggested upon analysis of gamma ray emissions that after the Big Bang, primordial mini black holes were formed. With Bardeen and Carter, he proposed the four laws of black hole mechanics, drawing an analogy with thermodynamics. In 1974, he calculated that black holes should thermally create and emit subatomic particles, known today as Bekenstein-Hawking radiation, until they exhaust their energy and evaporate.

In collaboration with Jim Hartle, Hawking developed a model in which the universe had no boundary in space-time, replacing the initial singularity of the classical Big Bang models with a region akin to the North Pole: one cannot travel north of the North Pole, as there is no boundary. While originally the no-boundary proposal predicted a closed universe, discussions with Neil Turok led to the realisation that the no-boundary proposal is also consistent with a universe which is not closed.

Along with Thomas Hertog at CERN, in 2006 Hawking proposed a theory of "top-down cosmology," which says that the universe had no unique initial state, and therefore it is inappropriate for physicists to attempt to formulate a theory that predicts the universe's current configuration from one particular initial state. Top-down cosmology posits that in some sense, the present "selects" the past from a superposition of many possible histories. In doing so, the theory suggests a possible resolution of the fine-tuning question: It is inevitable that we find our universe's present physical constants, as the current universe "selects" only those past histories that led to the present conditions. In this way, top-down cosmology provides an anthropic explanation for why we find ourselves in a universe that allows matter and life, without invoking an ensemble of multiple universes.

Hawking's many other scientific investigations have included the study of quantum cosmology, cosmic inflation, helium production in anisotropic Big Bang universes, large N cosmology, the density matrix of the universe, topology and structure of the universe, baby universes, Yang-Mills instantons and the S matrix, anti de Sitter space, quantum entanglement and entropy, the nature of space and time, including the arrow of time, spacetime foam, string theory, supergravity, Euclidean quantum gravity, the gravitational Hamiltonian, Brans-Dicke and Hoyle-Narlikar theories of gravitation, gravitational radiation, and wormholes.

At a George Washington University lecture in honour of NASA's fiftieth anniversary, Hawking theorised on the existence of extraterrestrial life, believing that "primitive life is very common and intelligent life is fairly rare."

Losing an old bet

Hawking was in the news in July 2004 for presenting a new theory about black holes which goes against his own long-held belief about their behaviour, thus losing a bet he made with Kip Thorne and John Preskill of Caltech. Classically, it can be shown that information crossing the event horizon of a black hole is lost to our universe, and that thus all black holes are identical beyond their mass, electrical charge and angular velocity (the "no hair theorem"). The problem with this theorem is that it implies the black hole will emit the same radiation regardless of what goes into it, and as a consequence that if a pure quantum state is thrown into a black hole, an "ordinary" mixed state will be returned. This runs counter to the rules of quantum mechanics and is known as the black hole information paradox.

Human spaceflight

At the fiftieth anniversary of NASA in 2008, Hawking gave a keynote speech on the final frontier exhorting and inspiring the space technology community on why we (the human race) explore space.

At the celebration of his sixty-fifth birthday on 8 January 2007, Hawking announced his plan to take a zero-gravity flight in 2007 to prepare for a sub-orbital spaceflight in 2009 on Virgin Galactic's space service. Billionaire Richard Branson pledged to pay all expenses for the latter, costing an estimated £100,000. Stephen Hawking's zero-gravity flight in a "''Vomit Comet''" of Zero Gravity Corporation, during which he experienced weightlessness eight times, took place on 26 April 2007. He became the first quadriplegic to float in zero-gravity. This was the first time in forty years that he moved freely, without his wheelchair. The fee is normally US$3,750 for 10–15 plunges, but Hawking was not required to pay the fee. A bit of a futurist, Hawking was quoted before the flight saying: }} In an interview with ''The Daily Telegraph'', he suggested that space was the Earth's long term hope. He continued this theme at a 2008 Charlie Rose interview.

Existence and nature of extraterrestrial life

Hawking has indicated that he is almost certain that alien life exists in other parts of the universe and uses a mathematical basis for his assumptions. "To my mathematical brain, the numbers alone make thinking about aliens perfectly rational. The real challenge is to work out what aliens might actually be like." He believes alien life not only certainly exists on planets but perhaps even in other places, like within stars or even floating in outer space. He also warns that a few of these species might be intelligent and threaten Earth. Contact with such species might be devastating for humanity. "If aliens visit us, the outcome would be much as when Columbus landed in America, which didn't turn out well for the Native Americans," he said. He advocated that, rather than try to establish contact, humans should try to avoid contact with alien life forms.

Illness

Stephen Hawking is severely disabled by a motor neurone disease known as Amyotrophic lateral sclerosis (ALS), sometimes known as Lou Gehrig's disease. Hawking's illness is markedly different from typical ALS because if confirmed, Hawking's case would make for the most protracted case ever documented. A survival for more than ten years after diagnosis is uncommon for ALS; the longest documented durations, other than Hawking's, are 32 and 39 years and these cases were termed benign because of the lack of the typical progressive course.

When he was young, he enjoyed riding horses. At Oxford, he coxed a rowing team, which, he stated, helped relieve his immense boredom at the university. Symptoms of the disorder first appeared while he was enrolled at University of Cambridge; he lost his balance and fell down a flight of stairs, hitting his head. Worried that he would lose his genius, he took the Mensa test to verify that his intellectual abilities were intact. The diagnosis of motor neurone disease came when Hawking was 21, shortly before his first marriage, and doctors said he would not survive more than two or three years. Hawking gradually lost the use of his arms, legs, and voice, and as of 2009 has been almost completely paralysed.

During a visit to the research centre CERN in Geneva in 1985, Hawking contracted pneumonia, which in his condition was life-threatening as it further restricted his already limited respiratory capacity. He had an emergency tracheotomy, and as a result lost what remained of his ability to speak. He has since used an electronic voice synthesiser to communicate.

The DECtalk DTC01 voice synthesiser he uses, which has an American English accent, is no longer being produced. Asked why he has still kept it after so many years, Hawking mentioned that he has not heard a voice he likes better and that he identifies with it. Hawking is said to be looking for a replacement since, aside from being obsolete, the synthesiser is both large and fragile by current standards. As of mid 2009, he was said to be using NeoSpeech's VoiceText speech synthesiser.

In Hawking's many media appearances, he appears to speak fluently through his synthesiser, but in reality, it is a tedious drawn-out process. Hawking's setup uses a predictive text entry system, which requires only the first few characters in order to auto-complete the word, but as he is only able to use his cheek for data entry, constructing complete sentences takes time. His speeches are prepared in advance, but having a live conversation with him provides insight as to the complexity and work involved. During a TED Conference talk, it took him seven minutes to answer a question.

He describes himself as lucky, despite his disease. Its slow progression has allowed him time to make influential discoveries and has not hindered him from having, in his own words, "a very attractive family." When his wife, Jane, was asked why she decided to marry a man with a three-year life expectancy, she responded, "Those were the days of atomic gloom and doom, so we all had a rather short life expectancy." On 20 April 2009, Cambridge University released a statement saying that Hawking was "very ill" with a chest infection, and was admitted to Addenbrooke's Hospital. The following day, it was reported that his new condition was "comfortable" and he would make a full recovery from the infection.

As popular science advocate

Hawking has played himself on numerous television shows and has been portrayed in many more. He has played himself on a ''Red Dwarf'' anniversary special, played a hologram of himself on the episode "Descent" of ''Star Trek: The Next Generation'', appeared in a skit on ''Late Night with Conan O'Brien'', and appeared on the Discovery Channel special ''Alien Planet''. He has also played himself in several episodes of ''The Simpsons'' and ''Futurama'', and has had an action figure made of his ''Simpsons'' likeness. In 2008, Hawking was the subject of and featured in the documentary series ''Stephen Hawking, Master of the Universe'' for Channel 4. In September 2008, Hawking presided over the unveiling of the 'Chronophage' (time-eating) Corpus Clock at Corpus Christi College Cambridge. His actual synthesiser voice was used on parts of the Pink Floyd song "Keep Talking" from the 1994 album ''The Division Bell'', as well as on Turbonegro's "Intro: The Party Zone" on their 2005 album ''Party Animals'', Wolfsheim's "Kein Zurück (Oliver Pinelli Mix)".

Recognition

Acclaim

On 19 December 2007, a statue of Hawking by renowned late artist Ian Walters was unveiled at the Centre for Theoretical Cosmology, University of Cambridge. In May 2008, the statue of Hawking was unveiled at the African Institute for Mathematical Sciences in Cape Town. The Stephen W. Hawking Science Museum in San Salvador, El Salvador is named in honour of Stephen Hawking, citing his scientific distinction and perseverance in dealing with adversity. Stephen Hawking Building in Cambridge opened on 17 April 2007. The building belongs to Gonville and Caius College and is used as an undergraduate accommodation and conference facility.

Distinctions

Hawking's belief that the lay person should have access to his work led him to write a series of popular science books in addition to his academic work. The first of these, ''A Brief History of Time'', was published on 1 April 1988 by Hawking, his family and friends, and some leading physicists. It surprisingly became a best-seller and was followed by ''The Universe in a Nutshell'' (2001). Both books have remained highly popular all over the world. A collection of essays titled ''Black Holes and Baby Universes'' (1993) was also popular. His book, ''A Briefer History of Time'' (2005), co-written by Leonard Mlodinow, aims to update his earlier works and make them accessible to an even wider audience. In 2007 Hawking and his daughter, Lucy Hawking, published ''George's Secret Key to the Universe'', a children's book focusing on science that has been described as "like ''Harry Potter'', but without the magic." The book includes information on Hawking radiation.

Hawking supports the children's charity SOS Children's Villages UK.

Awards and honours

1975 Eddington Medal

1976 Hughes Medal of the Royal Society

1979 Albert Einstein Medal

1981 Franklin Medal

1982 Order of the British Empire (Commander)

1985 Gold Medal of the Royal Astronomical Society

1986 Member of the Pontifical Academy of Sciences

1988 Wolf Prize in Physics

1989 Prince of Asturias Awards in Concord

1989 Companion of Honour

1999 Julius Edgar Lilienfeld Prize of the American Physical Society

2003 Michelson Morley Award of Case Western Reserve University

2006 Copley Medal of the Royal Society 2008 Fonseca Prize of the University of Santiago de Compostela 2009 Presidential Medal of Freedom, the highest civilian honour in the United States

Personal life

Hawking revealed that he did not see much point in obtaining a doctorate if he were to die soon. Hawking later said that the real turning point was his 1965 marriage to Jane Wilde, a language student. After gaining his PhD at Trinity Hall, he became first a Research Fellow, and later on a Professorial Fellow at Gonville and Caius College. Jane Hawking (née Wilde), Hawking's first wife, cared for him until 1991 when the couple separated, reportedly because of the pressures of fame and his increasing disability. They had three children: Robert, Lucy, and Timothy. Hawking then married his nurse, Elaine Mason (who was previously married to David Mason, the designer of the first version of Hawking's talking computer), in 1995. In October 2006, Hawking filed for divorce from his second wife amid claims by former nurses that she had abused him.

In 1999, Jane Hawking published a memoir, ''Music to Move the Stars'', detailing the marriage and his breakdown; in 2010 she published a revised version, ''Travelling to Infinity, My Life with Stephen''. Hawking's daughter, Lucy, is a novelist. Their oldest son, Robert, emigrated to the United States, married, and has a son. After a period of estrangement, Hawking and his first family were reconciled in 2007.

His view on how to live life is to "seek the greatest value of our action".

Hawking was asked about his IQ in a 2004 newspaper interview, and replied, "I have no idea. People who boast about their I.Q. are losers." Yet when asked "Are you saying you are not a genius?", Hawking replied "I hope I'm near the upper end of the range."

Hawking strongly opposed the US-led Iraq War, calling it "a war crime" and "based on lies". In 2004, he personally attended a demonstration against the war in Trafalgar Square, and participated in a public reading of the names of Iraqi war victims.

Religious views

In his early work, Hawking spoke of "God" in a metaphorical sense, such as in ''A Brief History of Time'': "If we discover a complete theory, it would be the ultimate triumph of human reason—for then we should know the mind of God." In the same book he suggested the existence of God was unnecessary to explain the origin of the universe. His 2010 book ''The Grand Design'' and interviews with the ''Telegraph'' and the Channel 4 documentary ''Genius of Britain'', clarify that he does "not believe in a personal God". Hawking writes, "The question is: is the way the universe began chosen by God for reasons we can't understand, or was it determined by a law of science? I believe the second." He adds, "Because there is a law such as gravity, the Universe can and will create itself from nothing."

His ex-wife, Jane, said during their divorce proceedings that he was an atheist. Hawking has stated that he is "not religious in the normal sense" and he believes that "the universe is governed by the laws of science. The laws may have been decreed by God, but God does not intervene to break the laws." In an interview published in ''The Guardian'' newspaper, Hawking regarded the concept of Heaven as a myth, stating that there is "no heaven or afterlife" and that such a notion was a "fairy story for people afraid of the dark."

Hawking contrasted religion and science in 2010, saying: "There is a fundamental difference between religion, which is based on authority, [and] science, which is based on observation and reason. Science will win because it works."

Selected publications

Technical

''Singularities in Collapsing Stars and Expanding Universes'' with Dennis William Sciama, 1969 Comments on Astrophysics and Space Physics Vol 1 No.1

''The Large Scale Structure of Space-Time'' with George Ellis, 1973, ISBN 0-521-09906-4

''The Nature of Space and Time'' with Roger Penrose, foreword by Michael Atiyah, New Jersey: Princeton University Press, 1996, ISBN 0-691-05084-8

''The Large, the Small, and the Human Mind'', (with Abner Shimony, Nancy Cartwright, and Roger Penrose), Cambridge University Press, 1997, ISBN 0-521-56330-5 (hardback), ISBN 0-521-65538-2 (paperback), Canto edition: ISBN 0-521-78572-3

''Information Loss in Black Holes'', Cambridge University Press, 2005

''God Created the Integers: The Mathematical Breakthroughs That Changed History'', Running Press, 2005 ISBN 0762419229

Popular

''A Brief History of Time'', (Bantam Press 1988) ISBN 055305340X

''Black Holes and Baby Universes and Other Essays'', (Bantam Books 1993) ISBN 0553374117

''The Universe in a Nutshell'', (Bantam Press 2001) ISBN 055380202X

''On The Shoulders of Giants. The Great Works of Physics and Astronomy'', (Running Press 2002) ISBN 076241698X

''A Briefer History of Time'', coauthored with Leonard Mlodinow, (Bantam Books 2005) ISBN 0553804367

''The Grand Design'', coauthored with Leonard Mlodinow, (Bantam Press 2010) ISBN 0553805371

Children's fiction

These are co-written with his daughter Lucy.

''George's Secret Key to the Universe'', (Random House, 2007) ISBN 9780385612708

''George's Cosmic Treasure Hunt'', (Simon & Schuster, 2009) ISBN 9781416986713

Films and series

''A Brief History of Time'' (1991)

''Stephen Hawking's Universe'' (1997)

''Horizon: The Hawking Paradox'' (2005)

''Masters of Science Fiction'' (2007)

''Stephen Hawking: Master of the Universe'' (2008) ''Into The Universe with Stephen Hawking'' (2010) A list of Hawking's publications through the year 2002 is available on his website.

See also

Basic concepts of quantum mechanics

Many-worlds interpretation, or flexiverse

Susskind–Hawking battle – see Black hole information paradox

References

Further reading

A layman's guide to Stephen Hawking

Ferguson, Kitty (1991). ''Stephen Hawking: Quest For A Theory of Everything''. Franklin Watts. ISBN 0-553-29895-X

Highly influential in the field. A much cited centennial survey.

Pickover, Clifford, ''Archimedes to Hawking: Laws of Science and the Great Minds Behind Them'', Oxford University Press, 2008, ISBN 978-0195336115

External links

Stephen Hawking's web site

Stephen Hawking's page on Academia.edu

The Life of Stephen Hawking – slideshow by ''The First Post''

Video: Stephen Hawking – discussion of two views of the universe

Videos: Stephen Hawking's concept of God, The role of God within the no boundary cosmology and Imaginary time

;Dated

Stephen Hawking says universe not created by God

Public Lectures, including debate with Roger Penrose 1996–2006

''Hawking celebrates own brief history'', 7 January 2002, BBC

"Leaping the Abyss", interview in ''Reason'' by Gregory Benford 2002-04-01

2002-03-24

''Black holes turned "inside out"'', 22 July 2004, BBC

''Return of the time lord'', Interview about "A Brief History of Time", 27 September 2005, The Guardian.

Stephen Hawking touches on God and science – Physicist says Pope John Paul told scientists not to study universe's origins msnbc June 2006

Press Release from the Catholic League on misquote of Pope by Hawking 2006-06-16

BBC interview 2008-12-05

Category:1942 births Category:Academics of the University of Cambridge Category:Adams Prize recipients Category:Albert Einstein Medal recipients Category:Alumni of Trinity Hall, Cambridge Category:Alumni of University College, Oxford Category:Calculating prodigies Category:Commanders of the Order of the British Empire Category:Cosmologists Category:English astronomers Category:English theoretical physicists Category:English science writers Category:Fellows of Gonville and Caius College, Cambridge Category:Fellows of the Royal Society Category:People educated at St Albans School, Hertfordshire Category:Honorary Fellows of University College, Oxford Category:Living people Category:Lucasian Professors of Mathematics Category:Members of the Department of Applied Mathematics and Theoretical Physics Category:Members of the United States National Academy of Sciences Category:Members of the Order of the Companions of Honour Category:Members of the Pontifical Academy of Sciences Category:People from Oxford Category:People from St Albans Category:People with motor neurone disease Category:Presidential Medal of Freedom recipients Category:Recipients of the Copley Medal Category:Recipients of the Gold Medal of the Royal Astronomical Society Category:Religious skeptics Category:Wolf Prize in Physics laureates Category:20th-century philosophers Category:21st-century philosophers Category:People educated at St Albans High School for Girls

af:Stephen Hawking ar:ستيفن هوكينج az:Stiven Hokinq bn:স্টিফেন হকিং zh-min-nan:Stephen Hawking be:Стывен Уільям Хокінг bg:Стивън Хокинг bs:Stephen Hawking ca:Stephen Hawking cs:Stephen Hawking cy:Stephen Hawking da:Stephen Hawking de:Stephen Hawking et:Stephen Hawking el:Στήβεν Χώκινγκ es:Stephen Hawking eo:Stephen Hawking ext:Stephen Hawking eu:Stephen Hawking fa:استیون هاوکینگ fr:Stephen Hawking ga:Stephen Hawking gl:Stephen Hawking ko:스티븐 호킹 hi:स्टीफन हॉकिंग hr:Stephen Hawking io:Stephen Hawking id:Stephen Hawking is:Stephen Hawking it:Stephen Hawking he:סטיבן הוקינג jv:Stephen Hawking kn:ಸ್ಟೀಫನ್‌ ಹಾಕಿಂಗ್ ka:სტივენ ჰოუკინგი ht:Stephen Hawking la:Stephanus Hawking lv:Stīvens Hokings lb:Stephen Hawking lt:Stephen Hawking hu:Stephen Hawking mk:Стивен Хокинг ml:സ്റ്റീഫൻ ഹോക്കിങ് mr:स्टीफन हॉकिंग ms:Stephen Hawking mn:Стивен Хокинг my:စတီဖင်ဟော့ကင်း nah:Stephen Hawking nl:Stephen Hawking ja:スティーヴン・ホーキング no:Stephen Hawking nn:Stephen Hawking pnb:سٹیفن ہاکنگ pl:Stephen Hawking pt:Stephen Hawking ro:Stephen Hawking ru:Хокинг, Стивен Уильям sq:Stephen Hawking simple:Stephen Hawking sk:Stephen Hawking sl:Stephen Hawking sr:Стивен Хокинг sh:Stephen Hawking fi:Stephen Hawking sv:Stephen Hawking tl:Stephen Hawking ta:ஸ்டீபன் ஹோக்கிங் te:స్టీఫెన్ హాకింగ్ th:สตีเฟน ฮอว์คิง tg:Стивен Ҳокинг tr:Stephen Hawking uk:Стівен Гокінг ur:سٹیفن ہاکنگ vi:Stephen Hawking war:Stephen Hawking yi:סטיווען האקינג zh-yue:霍金 bat-smg:Stephen Hawking zh:史蒂芬·霍金